This invention is related generally to the art of the construction and operation of lasers, and specifically to solid state pulse lasers that are Q-switched and injection seeded.
The basic structure common to nearly all lasers is well known. A light amplifying medium is positioned between two mirrors that forms a cavity between them that is resonant to oscillations of light having wavelength desired to be produced. An output beam of the laser is usually derived through one of the mirrors, in which case it is made to be only partially reflective, or around one of the mirrors, in which case it is made totally reflective. The light amplifying material may be in a gaseous, liquid or solid form, depending upon the type of laser and its specific desired characteristics. Lasers are also made to either operate with a continuous wave (c.w.) light output, or are designed to provide repetitive light pulses, the structure of each of these types of lasers being quite different.
A pulsed laser most commonly utilizes light amplifying material in the form of a rod. The most common material for the rod neodymium doped yttrium aluminum garnet (Nd:YAG). Energy is stored in the rod from a pulse of broad-band light from a flash lamp. That energy is subsequently released as a pulse of laser light by the opening of a Q-switch (shutter) that is positioned within the resonant cavity between the rod and one of its end mirrors.
A great deal of attention has been paid to designing the resonant cavity to oscillate in a few of the large number of transverse and longitudinal modes that a basic cavity and given light amplifying material will support. Oscillation in a single transverse mode is desired, the fundamental transverse mode being preferred because of its relatively uniform intensity profile across the laser beam within the cavity and thus in the output beam as well. A single transverse mode is also selected so that the oscillation may be limited to a single longitudinal mode, since each transverse mode supports oscillation in a large number of longitudinal modes. Because each longitudinal mode oscillates at a different frequency, a single longitudinal mode is selected in order to minimize the bandwidth, and thus maximize the coherence, of the output laser beam.
A single longitudinal mode can be selected by including a narrow band optical filter within the resonant cavity. Recently, however, a much more effective technique for selecting a single longitudinal mode has been developed and commercialized. This technique includes the injection of a continuous wave beam from an external, low power laser, into the cavity to seed the light amplifying material, commonly called an "injection seeding" technique.
The most commonly utilized laser oscillator resonators are classified int one of two types. A "stable" resonator is the simplest, having its end mirrors shaped to limit the size of the beam within the resonant cavity. Oscillation is generally limited to the fundamental, single transverse mode by the use of a pin hole within the cavity. The laser output beam has a highly uniform intensity profile across it, a very desirable characteristic. However, the controlled size of the laser beam within the oscillator is very small unless the resonant cavity is made to be much longer than usually desired. A small beam necessarily interacts with only a limited volume of the light amplifying material. The result is a laser output beam with limited energy. However, its intensity profile is highly uniform, so stable resonators are often used where a high energy output is not so important. An injection seeded, Q-switched solid state laser utilizing a resonant cavity provides a very good quality beam but requires a substantial amount of amplification of the output beam pulses for most applications.
The second general type of laser oscillator resonator is referred to as an "unstable" type. In this type, the end mirrors of the resonant cavity do not limit the cross-sectional size of the laser beam oscillating within it but rather allow the cross-sectional size of the beam to increase during each pass of the beam in one round trip through the resonator. The size of the beam is usually limited in such lasers, either by the outer surface of the laser rod, in solid state systems, or by some other abrupt aperture. Such a resonator has an advantage of developing a higher energy output beam because it interacts with a full volume of the laser rod. However, it has the disadvantage of providing a laser output beam with poor intensity uniformity across it because of the edge diffraction effects of the abrupt aperture within the resonant cavity.
The injection seeding technique is becoming the prevalent one for limiting longitudinal modes within a Q-switched pulsed laser. However, injection seeding requires a higher degree of transverse mode discrimination within the resonant cavity than do other types of lasers. Therefore, it is a primary object of the present invention to provide a laser resonant cavity structure having a high degree of transverse mode discrimination, while at the same time being capable of generating high energy light pulses at its output and producing a laser output beam having a uniform intensity distribution profile.